1. Field of the Invention
The invention relates to a bearing device for a wheel (hereinafter, may be simply referred to as “bearing device”), and a method of manufacturing the same. More specifically, the invention relates to a bearing device in which a wheel-mounting flange is formed on a peripheral surface of an inner shaft of an inner ring member, and an inner ring constituent member, which constitutes the inner ring member together with the inner shaft, is fitted to a periphery of the inner shaft, and a method of manufacturing the same.
2. Description of the Related Art
In a bearing device, heat treatment, such as induction hardening, is performed on a surface of a member, to increase surface hardness thereby improving abrasion resistance, and to reduce the possibility that fatigue failure may occur. For example, in a bearing device described in Japanese Patent Application Publication No. 2004-278673 (JP-A-2004-278673), the induction hardening is performed on a substantially entire peripheral surface of an inner shaft from a base of a wheel-mounting flange to an inner end portion in a vehicle-width direction. After performing the heat treatment on the peripheral surface of the inner shaft, abrasive finishing is generally performed on the peripheral surface of the inner shaft.
However, when the abrasive finishing is employed, the cost of installing an abrasive machine is high. This makes it difficult to reduce the cost of the product. In this regard, it is conceivable to finish the surface of the product by lathe turning. However, for example, when the inner shaft includes a spline portion, the peripheral surface of the inner shaft cannot be entirely heat-treated, because the spline portion may be strained due to the heat treatment if the peripheral surface of the inner shaft is entirely heat-treated; In this case, if a portion of the peripheral surface of the inner shaft is heat-treated, and the peripheral surface of the inner shaft is finished by lathe turning at a constant tip speed, a raw portion, which is a non-heat-treated portion, is cut by a larger amount than an amount by which the heat-treated portion is cut. Therefore, a step is formed at a border between the non-heat-treated portion and the heat-treated portion. This may make it difficult to fit a member to a periphery of the inner shaft at the non-heat-treated portion with a sufficient interference, and to firmly fix the member to the inner shaft. Also, a contact width between the member and the inner shaft (i.e., a length of a contact portion between the member and the inner shaft in an axial direction) may be short, and therefore, a creep may occur in the member.